Polycystic Kidney Diseases (PKD) are the leading cause of end-stage renal failure and are characterized by the development of renal cysts along the entire length of the nephron. They require extensive treatments, such as dialysis and kidney transplantation. Only limited forms of therapy for PKD exist, since the molecular mechanisms underlying the formation of renal cysts are still poorly understood. Over the years, considerable progress has been made in identifying genes mutated in human forms of PKD and in the development of animal models to study the pathogenesis of these detrimental diseases. Besides mouse and rat PKD models, the more primitive pronephric kidney of Xenopus or zebrafish has emerged as an alternative model system to study the molecular mechanisms underlying the epithelial malformations causing PKD. With its fast development and ease of molecular manipulations, the pronephric kidney is an ideal companion system to the study of PKD in humans or mice. In this proposal we will use the mouse metanephros and the amphibian pronephros to study the RNA binding molecule Bicaudal-C. Mice lacking Bicaudal-C protein develop renal cysts as early as embryonic day 15.5. Cyst formation is first detected in the glomerulus, but can later be detected along the entire length of the nephron. Similarly, in Xenopus, loss-of-Bicaudal-C induces a PKD-like phenotype in the pronephros. Importantly, the molecular mechanism of Bicaudal-C activity in kidney development and its connection to the genes mutated in human PKD, i.e. Polycystin-1, Polycystin-2 and Polyductin/Fibrocystin is still not understood. This proposal will address these questions. We will test the hypothesis that Bicaudal-C is a translational regulator of genes involved in Polycystic Kidney Disease. It is based on four observations: (1) Bicaudal-C mutant mice have reduced Polycystin-2 mRNA and protein levels before the onset of cyst formation. (2) The regulation of Polycystin-2 is posttranscriptional. (3) Bicaudal-C protein is localized to P-Bodies and has been shown in Drosophila and C. elegans to regulate a selected group of mRNAs at the posttranscriptional level. (4) Many PKD genes have evolutionary conserved miRNA binding sites in their 3' UTR. If successful, this study will provide novel insights to the underlying biological and biochemical pathways leading to the renal cyst formation in PKD. It will integrate one of the least understood PKD genes into the existing paradigms of PKD. As such, it will be directly applicable to future studies of PKD and may provide a new angle for therapeutic interventions.